I have both and think they are the reason for my off-idle stumble. Not that it's their fault, just I don't have them tuned right yet. Both are designed to improve drivability and idle for street-driven vehicles. A lot of factors come into play like cam/engine profile. The centrifugal advance kit comes with 3 different sets of spring tensions, and the adjustable vac advance controls how quickly the advance reacts to vac drop, or case pressure change. I ordered an advance timing light so I can get it right. The vac advance came with instructions recommending turning the adjuster 1 turn clockwise then test drive after each adjustment till spark knock is noticed, then back it off counter-clockwise 1 turn. On the centrifugal advance kit, lighter springs make the advance come on sooner, the heavier springs delay it to higher RPM. Usually the stock springs (or middle weight springs) end up working just fine. For my mild performance engine with Vortecs, the Performer cam calls for 12 degrees initial timing, and the Vortec heads like 32 degrees total advance. Regular Chevy heads are more like 34-38. Most racers will eliminate the vac advance altogether, and remove the centrifugal advance springs and weights completely and wire the advance down so it don't move. This sets your distributor to use no advance. Your initial timing may have to be higher in this configuration so you can achieve higher total timing when you're up on the RPMs. It's a delicate balance though, too much initial timing will give you spark knock/detonation which can damage parts.
